The immune system

You are surrounded by germs all day, yet most of the time you stay well. That is your immune system working, and it comes in two layers.

The first layer is fast and general. It is your skin and other barriers, plus cells that patrol your body and simply eat anything that looks like an invader. It does not care exactly what the germ is. It just attacks quickly. When a cut goes red, warm and swollen, that is this layer switching on.

The second layer is slower but much cleverer. It learns the exact shape of the germ and builds antibodies, tiny locks made to grab that one shape and no other. It takes a few days the first time, because the body has to find the right cell and let it multiply. Once the germ is beaten, some of those cells stay behind as memory. Next time the same germ shows up, the response is so fast you may never feel ill.

That memory is the whole trick behind vaccines. A vaccine shows your body a harmless version or piece of a germ, so you build the antibodies and the memory without ever getting sick. Then if the real thing arrives, you are already ready.

Immunity has two arms that work together: innate and adaptive.

The innate arm is the front line, and it has no memory. Physical barriers like skin and mucus come first. Behind them are phagocytes (macrophages and neutrophils) that engulf invaders, the complement system that punches holes in bacteria and flags them for eating, and inflammation that brings all of this to the site. It responds the same way to almost anything, within minutes to hours.

The adaptive arm is specific and it remembers. It runs on lymphocytes. B cells make antibodies, proteins that bind a particular target and either neutralise it or tag it for destruction. T cells split into jobs: helper T cells coordinate the response, and cytotoxic T cells kill the body's own cells once a virus has hijacked them. The shape that all of this recognises is called an antigen.

The key idea is clonal selection. Long before you ever meet a germ, your body already carries millions of B cells, each displaying one random receptor shape. When an antigen arrives, the few cells whose receptor happens to fit are switched on and told to divide. That one clone multiplies into an army, all making antibodies against that exact target. Nothing designed the right cell on demand. The right cell was already there and got picked out.

This is why the second meeting goes so differently. The first (primary) response is slow, because the matching clone starts rare and has to build up over days. Afterwards, memory cells linger. On a second exposure the secondary response is faster and stronger, often clearing the germ before you notice symptoms. That gap between primary and secondary is immunological memory, and it is exactly what a vaccine installs by showing you a safe version of the antigen first.

One line on the group level: if enough people are immune, a germ struggles to find a new host and even the unvaccinated are partly shielded. That is herd immunity.

Where the diversity comes from. A B or T cell needs a receptor for almost any shape it might one day meet, yet the genome is far too small to store them all. The answer is combinatorial. During development, V(D)J recombination cuts and pastes gene segments (variable, diversity and joining) more or less at random, and imprecise joining adds extra variation, so the body can generate on the order of \(10^{11}\) distinct receptors before it has seen a single pathogen. Once a response is under way, activated B cells run somatic hypermutation in their germinal centres and the best binders are selected in a round of affinity maturation, so the antibodies you finish with bind far more tightly than the ones you started with.

How the body decides what to show a T cell. T cells do not see free-floating antigen. They read short peptides held up on MHC molecules. Class I MHC sits on nearly every cell and displays fragments of what is being made inside, so cytotoxic (CD8) T cells can spot an infected or cancerous cell from within. Class II MHC is carried by professional antigen-presenting cells and shows fragments of what they have swallowed to helper (CD4) T cells. This presentation logic is why tissue matching matters for transplants.

Telling self from non-self. Because receptors are made at random, some will inevitably fit the body's own molecules. Central tolerance weeds many of these out as lymphocytes mature (in the thymus for T cells), and further checks act in the periphery. When that filtering fails, the immune system attacks the body itself, which is what an autoimmune disease is. The whole system is a balance between reacting hard enough to clear threats and staying blind to yourself.

The structure that does the binding. An antibody is a Y shape with constant regions that set its class and effector role and variable regions at the tips that do the recognising. The variable tips are the product of all that recombination and mutation; the constant stem is what other cells and the complement system latch onto.

The arms race. Fast-mutating pathogens change the very shapes your memory was built on. Influenza drifts a little every year, so last year's antibodies bind less well and you need an updated shot. This is the same selective logic that natural selection runs at the scale of whole populations, playing out here inside a single body: variation in receptors, selection of the ones that fit, and amplification of the winners. A vaccine simply lets that selection happen against a harmless stand-in first.